1.) Field of the Invention
The present invention relates to a thermal transfer image-receiving sheet. More particularly, the present invention relates to a thermal transfer image-receiving sheet capable of recording thereon sublimating dye images having an excellent clarity and gloss, at a high sensitivity.
2.) Description of the Related Arts
Currently there is an enormous interest in the development of dye thermal transfer type printers as a color hand copier for recording high quality colored images.
In the dye thermal transfer type printing system, an ink sheet composed of a base film and a yellow, cyan or magenta dye layer coated on a surface of the base film is superimposed on an image-receiving sheet composed of a substrate sheet and an image receiving resinous layer formed on a surface of the substrate sheet in such a manner that the dye layer of the ink sheet comes into direct contact with the image-receiving resinous layer of the image-receiving sheet, and the ink sheet is locally heated by heat applied by a thermal head of the printer, whereby portions of the dye in the ink sheet are thermally transferred to the image-receiving resinous layer to provide colored images. In this thermal transfer procedure of the colored images, the heating operation of the thermal head is continuously controlled in accordance with electrical signals corresponding to the images or pictures to be recorded, and the amount of dye transferred from the ink layer to the image-receiving layer is continuously controlled in accordance with the amount of heat and the heating time applied by the thermal head, to print continuous tone full color images having a desired color density 54 (darkness) on the image-receiving resinous layer.
In conventional dye image-receiving sheets, the dye image-receiving resinous layer comprises, as a principal component, a substrated copolyester resin having a high affinity to sublimating dyes.
A colored image-forming mechanism of the dye image thermal transfer printing system is described in "Shikizai (Coloring materials)", vol. 59, No. 10, 1986, pages 607. In this mechanism, when an image-receiving polyester resinous layer having a relatively low glass transition temperature is locally heated, vigorous molecular motions (vibrations) occur in portions of the polyester molecule chains of the polyester resin located in the heated portions of the image-receiving layer and these portions are melted or softened to form viscous liquid layer portions. Also, in the printing procedure, the dye in the ink layer is locally sublimated and the sublimated dye penetrates the melted or softened polyester resinous layer portions. When the polyester resinous layer portions are solidified by cooling, the penetrated dye is embedded in and fixed to the solidified amorphous portions of the polyester resinous layer portions, to thereby provide colored images.
Nevertheless, the colored images recorded by the dye thermal transfer printing system are disadvantageous in that such recorded images have a low color evenness and density (darkness), and a poor durability during storage.
To eliminate the above-mentioned disadvantages, an attempt has been made to increase the surface smoothness of the image-receiving resinous layer, to thereby enhance a close adhesion of the dye layer surface of the ink sheet to the image-receiving resinous layer surface of the image-receiving sheet, and thus raise the printing speed of the image-receiving sheet and improve the quality of the resultant colored images. For example, Japanese Unexamined Patent Publication No. 62-211,195 discloses an attempt to provide an image-receiving resinous layer having a high surface smoothness, by coating a substrate sheet surface with an aqueous coating liquid by a casting method. This attempt provides an image-receiving resinous layer with a high surface gloss but is disadvantageous in that, since the aqueous coating liquid layer formed on a cast drum surface is solidified by evaporating off water in the aqueous coating liquid layer, the resultant image-receiving resinous layer contains a number of pores derived from the evaporation of the water, and thus the surface smoothness and uniformity of the image-receiving resinous layer is not satisfactory and the resultant colored images on the image-receiving resinous layer exhibit an unsatisfactory color density (darkness) evenness.
Also, Japanese Unexamined Patent Publication No. 62-173,295 discloses an image-receiving resinous layer formed by coating a surface of a substrate sheet with a coating liquid containing a radical-polymerizable oligomer or monomer which can be used by an actinic ray irradiation, and applying the actinic ray irradiation to the coating liquid layer on the substrate sheet surface.
This technique is advantageous in that the resultant image-receiving resinous layer has a high gloss, but is disadvantageous in that the cured resin in the resultant image-receiving resinous layer is crosslinked at a high cross-linkage density, and thus cannot be melt-softened to form a viscous liquid layer when heated in the thermal transfer printing procedure, and therefore, the resultant image-receiving resinous layer exhibits a poor dye-receiving activity and it is difficult to record thereon clear colored images having a high color density (darkness).